That would be a function of your OS as much as of Thunderbird. Within Thunderbird, ctrl+n will do that. But you need some facility that will recognise the magic keystroke (regardless of what other applications you might have open), launch your mail client if it isn't already running (and somehow cope with it being immediately busy downloading email messages), and then pass it an instruction telling it to open a new message. That message is likely to be dependent on whichever email client you choose to make use of. Which of your accounts or identities should it use? Should the message be composed using html, or plain text? I use different email accounts for different purposes. A single default wouldn't suit my own workflow and habits. But I hope I have pointed out some of the obstacles.